Elizabeth R. Schermer

Geometry and kinematics of continental basement deformation during the Alpine orogeny, Mt. Olympos region, Greece

Abstract In the Hellenic Alps, continental crust of the leading edge of the Apulian plate was subducted beneath the European plate during Late Cretaceous-Early Tertiary closure of Tethyan ocean basin(s). Carbonate rocks of the external Hellenides are exposed in the Mt. Olympos tectonic window, overthrust by several thin (∼0.2–4 km thick) thrust sheets of granitic basement derived from the internal Hellenides. Field mapping and structural data from the Mt. Olympos region reported here establish the geometry, kinematics and strain history of the continental basement before, during and after continental collision and subduction. Seven deformational events document the change from ductile conditions obtained at depth in the early stages of deformation, to lower temperature ductile and semi-ductile shortening-related fabrics, to brittle extensional features related to exhumation of the metamorphic rocks. D 1 – D 4 occurred during subduction of basement nappes and are marked by thrust faulting and folding under blueschist facies conditions; D 5 – D 7 are related to unroofing of blueschist facies metamorphic rocks and are associated with extension. Each of the major shortening phases ( D 1 – D 3 ) verges toward the foreland (southwest). Oppositely-directed D 5 – D 7 extension occurred behind the active thrust belt during continued regional shortening. Normal faults have strongly modified the original thrust-related geometry in the Mt. Olympos region and elsewhere in the internal Hellenides.

Late Holocene surface ruptures on the southern Wairarapa fault, New Zealand: Link between earthquakes and the uplifting of beach ridges on a rocky coast

The Holocene beach ridges at Turakirae Head, New Zealand, are remarkable because the fault that caused their uplift is accessible to paleoseismic trenching. Based on 40 14 C samples from eight trenches, we identify five surface-rupturing earthquakes since ca. 5.2 ka (mean earthquake recurrence of 1230 ± 190 yr). The paleoearthquake record includes two more events than were recorded by the uplift and stranding of beach ridges at Turakirae Head. We conclude that beach ridges may provide an incomplete record of paleoearthquakes on oblique-reverse faults. The southern end of the Wairarapa fault includes several splays in the near surface at variable distances from Turakirae Head. Variable partitioning of slip between these splays (and perhaps the subduction interface down-dip of them) is inferred to have caused variable magnitudes of coseismic uplift at the coast, where at least one 14 C data support the view that a widespread post–Last Glacial Maximum aggradational terrace in southern North Island, New Zealand, was abandoned soon after 12.1 cal yr B.P. From this, we infer that the Wairarapa fault has a late Quaternary slip rate of 11 ± 3 mm/yr.

Late Cenozoic Structure and Tectonics of the Northern Mojave Desert

In the Fort Irwin region of the northern Mojave desert, late Cenozoic east striking sinistral faults predominate over northwest striking dextral faults of the same age. Kinematic indicators and offset marker units indicate dominantly sinistral strike slip on the east striking portions of the faults and sinistral-thrust slip on northwest striking, moderately dipping segments at the east ends of the blocks. Crustal blocks ∼7–10 km wide by ∼50 km long are bounded by complex fault zones up to 2 km wide at the edges and ends of each block. Faulting initiated after ∼11 Ma, and Quaternary deposits are faulted and folded. We document a minimum of 13 km cumulative sinistral offset in a north-south transect from south of the Bicycle Lake fault to north of the Drinkwater Lake fault. Paleomagnetic results from 50 sites reveal two direction groups in early and middle Miocene rocks. The north-to-northwest declinations of the first group are close to the middle Miocene reference pole. However, rock magnetic studies suggest that both primary and remagnetized directions are present in this group. The northeast declinations of the second group are interpreted as primary and 63.5° ± 7.6° clockwise from the reference pole and suggest net post middle Miocene clockwise rotation of several of the east trending blocks in the northeast Mojave domain. The Jurassic Independence Dike Swarm in Fort Irwin may be rotated 25–80° clockwise relative to the swarm north of the Garlock fault, thus supporting the inference of clockwise rotation. Using a simple-shear model that combines sinistral slip and clockwise rotation of elongate crustal blocks, we predict ∼23° clockwise rotation using the observed fault slip, or one-third that inferred from the paleomagnetic results. The discrepancy between slip and rotation may reflect clockwise bending at the ends of fault blocks, where most of our paleomagnetic sites are located. However, at least 25°–40° of clockwise tectonic rotation is consistent with the observed slip on faults within the domain plus possible “rigid-body” rotation of the region evidenced by clockwise bending of northwest striking domain-bounding faults. Our estimates of sinistral shear and clockwise rotation suggest that approximately half of the 65 km of dextral shear in the Eastern California Shear Zone over the last 10 m.y. occurred within the northeast Mojave Domain. The remainder must be accommodated in adjacent structural domains, e.g., east of the Avawatz Mountains and west of the Goldstone Lake fault. Supporting Appendices 1 and 2 are available on diskette or via Anonymous FTP from kosmos.agu.org, directory APEND (Username -- anonymous, Password = guest). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W, Washington, DC 20009 or by phone at 800-966-2481;

Aegean paleomagnetic inclination anomalies. Is there a tectonic explanation

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Active faults, paleoseismology, and historical fault rupture in northern Wairarapa, North Island, New Zealand

Abstract More than a decade of paleomagnetic research in the Aegean area, mainly by French investigators (C. Laj C. Kissel and their colleagues), has provided a valuable data base for the evaluation of Cenozoic Aegean tectonics. Systematic rotations are documented and have been interpreted by the original investigators in the light of the evolution of the Hellenic arc. However, less attention has been paid to the significance of the paleomagnetic inclinations, which also are anomalous. Twenty one out of 22 paleomagnetic studies performed to date within the general Aegean region have yielded paleolatitudes that are too shallow when compared with reference curves for either Africa or Eurasia, suggesting northward relative transport with respect to both continents. Sedimentary, volcanic and plutonic units are involved; ages range from Oligocene to Pliocene. Some of these apparent paleolatitude anomalies may merely be the result of inclination flattening in certain kinds of clastic sedimentary rocks. However, shallow inclinations in igneous rocks cannot be so easily dismissed. The explanation for these anomalously shallow inclinations does not appear to lie with the reference curves, because several other reference curves calculated in several different ways yield essentially the same result. We suggest that the paleomagnetic pattern may reflect real northward displacement of crustal blocks within the Aegean region, measured with respect to both Europe and Africa. The amount of displacement appears to increase systematically to the west. The pattern of local block rotations noticed by earlier investigators argues that the displacement did not involve a single coherent Aegean block, but rather a collection of smaller blocks moving quasi-independently. Geological consequences of northward relative displacement of the Aegean block ought to include mid-Tertiary and/or later compressional structures in the north (e.g., Rhodope) and extensional structures in the leading edge of the African plate.

Jurassic magmatism in the central Mojave Desert: Implications for arc paleogeography and preservation of continental volcanic sequences

Abstract Active faulting in the upper plate of the Hikurangi subduction zone, North Island, New Zealand, represents a significant seismic hazard that is not yet well understood. In northern Wairarapa, the geometry and kinematics of active faults, and the Quaternary and historical surface‐rupture record, have not previously been studied in detail. We present the results of mapping and paleoseismicity studies on faults in the northern Wairarapa region to document the characteristics of active faults and the timing of earthquakes. We focus on evidence for surface rupture in the 1855 Wairarapa (MW 8.2) and 1934 Pahiatua (MW7.4) earthquakes, two of New Zealands largest historical earthquakes. The Dreyers Rock, Alfredton, Saunders Road, Waitawhiti, and Waipukaka Faults form a northeast‐trending, east‐stepping array of faults. Detailed mapping of offset geomorphic features shows the rupture lengths vary from c. 7 to 20 km and single‐event displacements range from 3 to 7 m, suggesting the faults are capable of generating M >7 earthquakes. Trenching results show that two earthquakes have occurred on the Alfredton Fault since c. 2900 cal. BP. The most recent event probably occurred during the 1855 Wairarapa earthquake as slip propagated northward from the Wairarapa Fault and across a 6 km wide step. Waipukaka Fault trenches show that at least three surface‐rupturing earthquakes have occurred since 8290–7880 cal. BP. Analysis of stratigraphic and historical evidence suggests the most recent rupture occurred during the 1934 Pahiatua earthquake. Estimates of slip rates provided by these data suggest that a larger component of strike slip than previously suspected is occurring within the upper plate and that the faults accommodate a significant proportion of the dextral component of oblique subduction. Assessment of seismic hazard is difficult because the known fault scarp lengths appear too short to have accommodated the estimated single‐event displacements. Faults in the region are highly segmented, disconnected, and probably structurally immature, which implies that apparent geometric discontinuities at the surface may not be significant barriers to rupture propagation at depth and that the surface rupture record significantly under‐represents the seismic slip on faults in the region.

Paleogeographic and tectonic implications of Jurassic sedimentary and volcanic sequences in the central Mojave block

Exposures of Jurassic magmatic rocks in the west-central Mojave Desert provide insight into the changing character of volcanism throughout Jurassic time and the paleogeography of the continental arc. Middle Jurassic explosive volcanism (Lower Sidewinder volcanic series) resulted in collapse of multiple calderas. This was followed by north-south extension broadly coeval with batholith emplacement. Late Jurassic effusive volcanism (Upper Sidewinder volcanic series) appears to reflect transtension over a larger region during intrusion of the Independence dike swarm. The Lower Sidewinder volcanic series consists of a subaerial, nested caldera complex with an aggregate thickness of >4 km. The first caldera formed during the eruption of crystal-poor rhyolite ignimbrite. Outflow and intracaldera facies are intercalated with quartzose sandstone of probable cratonal provenance. The second caldera formed during the eruption of crystal- rich rhyolite to dacite ignimbrite. This ignimbrite exhibits complex mineralogical zoning and contains two units of probable collapse-related mesobreccia within the 1,400-m-thick intracaldera sequence. A third caldera, nested within the second, is filled with crystal-rich biotite-dacite ignimbrite and tuff breccia. A fourth caldera, largely coincident with the second, formed during eruption of lithic- and pumice-lapilli dacite ignimbrite. Eruption and collapse appear to have been multistage, as several units of reworked tuff and fallout tuff occur within the 1,750-m-thick intracaldera sequence, and caldera collapse breccias occur at the base and near the middle of the sequence. The top of this sequence contains reworked tuffs and epiclastic rocks, suggesting that the fourth caldera provided a posteruptive depocenter for accumulation of sediment. Normal faulting, tilting, and erosion followed explosive volcanism and was broadly contemporaneous with intrusion of Middle Jurassic porphyritic quartz monzonite plutons. Plutons and tilted Lower Sidewinder volcanic series are intruded and unconformably over-lain by latest Jurassic volcanic rocks (Upper Sidewinder volcanic series). The Upper Sidewinder volcanic series is characterized by alkalic basalt to basaltic andesite and rhyolite lavas, hypabyssal intrusions, and dikes, including the 148 Ma Independence dike swarm. Regional northeast-southwest extension is suggested by the presence of the northwest-striking dike swarm and by bimodal volcanism. The absence of debris-flow deposits and epiclastic rocks suggests an intra-arc region of low relief. Although local caldera-related subsidence was the principal control on accumulation of Lower Sidewinder volcanic rocks, preservation was likely enhanced by Middle Jurassic extension. Late Jurassic extension was of broader regional extent but lesser magnitude because it did not create basins or cause significant tilting of strata. The geometry, timing,and regional setting of Middle and Late Jurassic extension and magmatism suggest a sinistral oblique subduction regime for the Jurassic arc of the southern U.S. Cordillera.

Holocene earthquakes and right-lateral slip on the left-lateral Darrington-Devils Mountain fault zone, northern Puget Sound, Washington

Sedimentologic, stratigraphic, and geochronologic data from strata of early Mesozoic age in the central Mojave block elucidate the paleogeographic and tectonic evolution of the magmatic arc in the southern U.S. Cordillera. A sequence of calcareous siltstone, volcaniclastic conglomerate, tuff, and quartzose sandstone records the transition from shallow-marine rocks of the Fairview Valley Formation to the subaerial Sidewinder volcanic series. Quartzose sandstones occur below, within, and above the transitional sequence and indicate that texturally mature, craton-derived quartz sand gained access to the arc during the initial stages of volcanism. U-Pb data indicate that explosive volcanism began at 179.5 3.0 Ma and continued until 151 1.3 Ma (Lower Sidewinder volcanic series). A rhyolite dike of the Independence dike swarm (Upper Sidewinder volcanic series) that postdates normal faulting and tilting of the ignimbrites yielded a U-Pb date of 151.9 5.6 Ma. The data define the age of extension and development of the angular unconformity between the Upper and Lower Sidewinder volcanic series at ca. 151 Ma. The data suggest that at least part, and possibly all, of the Fairview Valley Formation is late Early Jurassic in age. We correlate the Fairview Valley Formation with Mesozoic metasedimentary rocks in the Rodman Mountains and Fry Mountains, and at Cave Mountain to the east. Eolian quartz arenites in these sequences suggest a coastal environment coeval with the Navajo Sandstone on the Colorado Plateau. The reinterpretation of the shallow-marine rocks as Jurassic instead of Triassic suggests a period of uplift and erosion or nondeposition extending from the Early Triassic into the Early Jurassic, followed by a return to marine conditions. Shallow-marine conditions persisted until the beginning of arc volcanism in the late Early Jurassic time. Similarities to the early Mesozoic arc of the Sierra Nevada, together with the structural evolution of the region, suggest that the change from high-standing to lowstanding paleogeography reflects a large-scale tectonic control on relative sea level related to a period of intra-arc extension or transtension. Schermer, E.R., Busby, C.J., and Mattinson, J.M., 2002, Paleogeographic and tectonic implications of Jurassic sedimentary and volcanic sequences in the central Mojave block, in Glazner, A.F., Walker, J.D., and Bartley, J.M., eds., Geologic Evolution of the Mojave Desert and Southwestern Basin and Range: Boulder, Colorado, Geological Society of America Memoir 195, p. 93–115. E.R. Schermer, C.J. Busby, and J.M. Mattinson 94

Holocene tectonics and fault reactivation in the foothills of the north Cascade Mountains, Washington

Sources of seismic hazard in the Puget Sound region of northwestern Washington include deep earthquakes associated with the Cascadia subduction zone, and shallow earthquakes associated with some of the numerous crustal (upper-plate) faults that crisscross the region. Our paleoseismic investigations on one of the more prominent crustal faults, the Darrington–Devils Mountain fault zone, included trenching of fault scarps developed on latest Pleistocene glacial sediments and analysis of cores from an adjacent wetland near Lake Creek, 14 km southeast of Mount Vernon, Washington. Trench excavations revealed evidence of a single earthquake, radiocarbon dated to ca. 2 ka, but extensive burrowing and root mixing of sediments within 50–100 cm of the ground surface may have destroyed evidence of other earthquakes. Cores in a small wetland adjacent to our trench site provided stratigraphic evidence (formation of a laterally extensive, prograding wedge of hillslope colluvium) of an earthquake ca. 2 ka, which we interpret to be the same earthquake documented in the trenches. A similar colluvial wedge lower in the wetland section provides possible evidence for a second earthquake dated to ca. 8 ka. Three-dimensional trenching techniques revealed evidence for 2.2 ± 1.1 m of right-lateral offset of a glacial outwash channel margin, and 45–70 cm of north-side-up vertical separation across the fault zone. These offsets indicate a net slip vector of 2.3 ± 1.1 m, plunging 14° west on a 286°-striking, 90°-dipping fault plane. The dominant right-lateral sense of slip is supported by the presence of numerous Riedel R shears preserved in two of our trenches, and probable right-lateral offset of a distinctive bedrock fault zone in a third trench. Holocene north-side-up, right-lateral oblique slip is opposite the south-side-up, left-lateral oblique sense of slip inferred from geologic mapping of Eocene and older rocks along the fault zone. The cause of this slip reversal is unknown but may be related to clockwise rotation of the Darrington–Devils Mountain fault zone into a position more favorable to right-lateral slip in the modern N-S compressional stress field.

Extensional arc setting and ages of Middle Jurassic eolianites, Cowhole Mountains (eastern Mojave Desert block, California)

We use LiDAR imagery to identify two fault scarps on latest Pleistocene glacial outwash deposits along the North Fork Nooksack River in Whatcom County, Washington (United States). Mapping and paleoseismic investigation of these previously unknown scarps provide constraints on the earthquake history and seismic hazard in the northern Puget Lowland. The Kendall scarp lies along the mapped trace of the Boulder Creek fault, a south-dipping Tertiary normal fault, and the Canyon Creek scarp lies in close proximity to the south-dipping Canyon Creek fault and the south-dipping Glacier Extensional fault. Both scarps are south-side-up, opposite the sense of displacement observed on the nearby bedrock faults. Trenches excavated across these scarps exposed folded and faulted late Quaternary glacial outwash, locally dated between ca. 12 and 13 ka, and Holocene buried soils and scarp colluvium. Reverse and oblique faulting of the soils and colluvial deposits indicates at least two late Holocene earthquakes, while folding of the glacial outwash prior to formation of the post-glacial soil suggests an earlier Holocene earthquake. Abrupt changes in bed thickness across faults in the Canyon Creek excavation suggest a lateral component of slip. Sediments in a wetland adjacent to the Kendall scarp record three pond-forming episodes during the Holocene—we infer that surface ruptures on the Boulder Creek fault during past earthquakes temporarily blocked the stream channel and created an ephemeral lake. The Boulder Creek and Canyon Creek faults formed in the early to mid-Tertiary as normal faults and likely lay dormant until reactivated as reverse faults in a new stress regime. The most recent earthquakes—each likely M w > 6.3 and dating to ca. 8050–7250 calendar years B.P. (cal yr B.P.), 3190–2980 cal. yr B.P., and 910–740 cal. yr B.P.—demonstrate that reverse faulting in the northern Puget Lowland poses a hazard to urban areas between Seattle (Washington) and Vancouver, British Columbia (Canada).